Method of making electric heating elements

ABSTRACT

An electric heating element is disclosed that is capable of cyclic operation at temperatures in excess of 2500* F in an air atmosphere. The heating element consists essentially of molybdenum wire as a substrate and a relatively thin layer of molybdenum disilicide over the substrate. A process consisting of heating the coils in a non-oxidizing atmosphere with boron silicide, sodium fluoride and silicon produces the heating elements under controlled time and temperature conditions.

United States Patent 1 Pepino et al.

METHOD OF MAKING ELECTRIC HEATING ELEMENTS Inventors: George T. Pepino, Huntington;

Seymour Priceman, Seaford, both of N.Y.

De Wiant Corporation, Detroit, Mich.

Filed: Dec. 30, 1971 Appl. No.: 214,486

Related US. Application Data Division of Ser. No. 87,358, Nov. 5, 1970, abandoned,

Assignee:

US. Cl 29/611, 29/610, 117/128, ll7/135 .1,117/231,117/D1G.10,l48/l3.1,

- 317/98 Int. Cl. 1105b 3/00 Field of Search 29/611, 610; 117/231, 127, 128, 135.1, DIG. 10;:117/98; 219/553, 270; 338/296, 269; 148/l3.1

References Cited UNITED STATES PATENTS 11/1949 Cohn 317/98 X Primary Examiner-Charles W. Lanham Assiria nt Examiner-Victor A. DiPalma Attorney Norinan J. UMalley and Donald R. Castle [57] ABSTRACT An electric heating element is disclosed that is capable of cyclic operation at temperatures in excess of 2500 F in an air atmosphere. The heating element consists essentially of molybdenum wire as a substrate and a relatively thin layer of molybdenum disilicide over the substrate. A process consisting of heating the coils in a non-oxidizing atmosphere with boron silicide, sodium fluoride and silicon produces the heating elements under controlled time and temperature conditions.

4 Claims, No Drawings METHOD OF MAKING ELECTRIC HEATING ELEMENTS CROSS-REFERENCE TO RELATED APPLICATION This application is a division of Ser. No. 87,358, filed Nov. abandoned, Nov. 5, 1970 and assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION This invention relates to electric heating elements for high temperature cyclic operation. More particularly, it relates to a low cost, boron-modified molybdenum disilicide-coated, molybdenum heating element.

Often, electronic and electrical components such as semiconductors and reed switches are encapsulated in glass. In the sealing operation an external electric heating element is commonly used to provide heat to soften the glass to form a seal. The heating element, therefore, must be capable of operating in air at temperatures in excess of 2,500 F in order to produce seals quickly and economically. Because of the intermittent heating and cooling, the high temperatures employed and the air atmosphere, there are only a few materials that can be used as a heating element.

Heretofore, in most instances, platinum and platinum alloy coils have been used as the heating elements. Although these coils have been the most satisfactory of any materials heretofore glass investment available, there are several disadvantages to the use of platinum. For example, in addition to being expensive, platinum is relatively weak at elevated temperatures. The coil of wire tends to sag and can touch the glass during the heating operation. A number of adverse results can occur. For instance, the electrical circuit can be shorted cutting off the heat. Also, if the glass has softened the coil can become embedded in the lgass thereby ruining the article being fabricated. The sagging can be prevented by encapsulating the coil in a ceramic, however, subsequent operation sometimes results in spalling and chips of the ceramic because of different expansion characteristics. The coil then becomes exposed to air and burns out. Additionally, because of the high cost of platinum the original cost of the heating element is large and recovery and reclamation is necessary. The investemnt in inventory of coils is relatively high.

It is believed, therefore, a heating element that can withstand the severe operating conditions better than platinum and is relatively inexpensive when compared to platinum is an advancement in the art. It is further believed that a relatively simple process for producing the heating elements would be an advancement in the art.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of this invention to provide an electric heating element capable of cyclic operation at rela-' tively high temperature in an air atmosphere.

It is an additional object of this invention to provide a process for the manufacture of the heating elements.

In accordance with one aspect of this invention, there is provided an electric heating element capable of operation under the before-mentioned severe condition.

The element consists essentially of a core of molybdenum wire of a length and diameter suitable for electric heating; the wire acts as a substrate for a relatively thin coating of boron modified molybdenum disilicide that is adhered to the coil.

In accordance with an additional aspect of this invention there is provided a process for manufacturing the beforementioned heating elements. The process comprises a. forming a mixture of boron silicide (8 8i) and sodium fluoride in specific weight ratios,

b. forming a layer of the mixture in a heat resistant container,

c. adding a layer of silicon powder,

(1. placing at least one uncoated heating element on top of said silicon powder,

e. covering said element with silicon powder,

f. heating the resulting container in a hydrogen atmosphere under controlled temperature conditions for a sufficient time to provide a boron modified molybdenum disilicide coating of sufficient thickness over the heating element.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above description of some of the aspects of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For brevity of the description of the heating element the geometric configuration will be referred to as a coil although other configurations can be used such as serpentine, loops, helical, concentric circles or any other shapes which are known to those skilled in the art.

As previously mentioned, one of the desired characteristics of the particular heating elements is the capability of operating at temperatures in excess of 2,500 F and up to 3,000 F. These temperatures are required in order to provide sufficient heat to soften the glass so that the seals can be made in very short time intervals (10-30 seconds typically). The heating elements of this invention because of the high melting point of the molybdenum core (4,730 F) enables high temperature operations. Platinum, previously used, has a considerably lower melting point (3,202F) thus, at the upper ranges of operation, the platinum coils tend to sag. Use of unprotected molybdenum, however, is not feasible in oxidizing atmospheres since it readily oxidizes at elevated temperatures. The foregoing disadvantages of molybdenum are over-come by providing a reatively thin layer of greater than about 1 mil thickness of a boron-modified molybdenum disilicide. Although thickness of coatings greater than about 5 mils can be achieved, it is believed that optimum results are obtained with coatings having a thickness of 3-5 mils.

The coating is formed during the heating cycle by reaction of the molybdenum with silicon and boron. Although the reactions are not known with centainty, it is believed that the following reactions occur:

1. NaF is thermally decomposed 2NaF 2 Na F, (gas) I 2. The fluorine gas reacts with the silicon to form silicon tetrafluoride 2F, Si SiF,

3. The silicon tetrafluoride reacts with the molybdenum surface 2SiF Mo MoSi 4 F (gas) A small amount, less than 1 percent of boron, is transferred to the surface by similar reactions.

Hydrogen gas as the furnace atmosphere is preferably used to provide a convenient and inexpensive nonoxidizing atmosphere for the parts and coating materials. It also reduces any residual oxides on molybdenum parts. An inert gas such as argon or helium can also be used to provide the non-oxidizing atmosphere.

The uncoated heating element is prepared by fabricating a wire generally having a diameter of from about 0.010 to about 0.50 inches, into the desired shape, such as a coil having a diameter of from about one-eighth inch to about 12 inches. Any foreign material is removed from the wire by any suitable means such as vaporization. The foregoing steps can be conducted by the supplier of the uncoated coil, although generally it is preferred to clean the coil at the site of the subsequent coating operation.

A furnace supplied with hydrogen is the major piece of equipment necessary. Containers or retorts that will withstand the subsequent heating are used. Graphite is a suitable material for retorts where many reuses are required. For a single usage, plain steel retorts can be used.

A layer of a mixture of boron silicide and sodium fluoride powder equivalent to I-5 percent (typically 1.5 percent) of the total weight of silicon in the retort and having a weight ratio of boron silicide to sodium fluoride of from about 1:1 to about 1:3 (preferably 1:2) is placed in the bottom of the container. This layer is cov ered with a layer of about one-fourth to about one-half inches of silicon powder. At least one coil, but preferably the maximum number that can be placed without touching each other, are placed over the silicon. A layer of silicon powder is added to cover the coils. If desired, additional alternate layers of coils and silicon can be placed in the container. The container is placed in the furnace and heated under a hydrogen atmosphere at a temperature of at least about 1,600 F for at least about 4 hours. Higher temperatures up to about 2,500 F can be used if desired, but a temperature of about 1,900" P and a time of about 8 hours is preferred since it is convenient and effective and places less rigorous demands on the furnace and the retorts.

After heating the container is removed from the furnace and the coils are separated from the powder. Thereafter, the coils are cleaned to remove materials adhering to the surface.

To explain this invention in greater detail, the following detailed example is presented. All parts, proportions and percentages are by weight unless otherwise indicated.

EXAMPLE I Molybdenum wire having a diameter of about 0.020 inches is formed into a number of coils having about one-fourth inch diameter and having a length of about three-eighths inch. Each end of the coil extends about one-half inches to enable subsequent connection to an electrical supply. The coil is cleaned by vapor degreasing in perchloroethylene.

The coils are batch cleaned by vapor degreasing in perchloroethylene. A blended mixture consisting of 5 grams of boron silicide (B Si) powder and 10 grams of silicon fluoride (NaF) powder is evenly distributed on the bottom of a rectangular graphite boat or retort approximately 5 inches wide by 11 inches long by l inches deep having a wall thickness of about one-half inch.

A one-half inch thick layer of silicon metal powder (-20 X 60 mesh) is placed on top of the NaF and B Si mixture. About 250 coils are laid out on the top of this silicon powder so they do not touch each other and an additional one-half inch thick layer of silicon is poured over the layer of coils. A second layer of about two hundred and fifty coils is laid out on the surface and these in turn are covered with about one-half inch of silicon. The total weight of silicon to fill the described retort or boat is about 900 grams. The filled boat is covered with a flat, loose graphite cover and placed in the cooling chamber of an inconel muffle controlled atmosphere furnace which is heated externally by silicon carbide electrical resistance elements. Several such boats can be processed together. When the boats are initially placed in the entry chamber an argon atmosphere is flowing in the muffle and cooling chamber. After flowing at 15 CFM for 10 minutes to allow all air to be purged from the retorts the furnace atmosphere is changed to dry hydrogen gas flowing at about CFM. After an additional 5 minutes, which is generally sufficient to replace the argon atmosphere in the boats with the hydrogen atmosphere, the boats are stoked into the hot zone which has been previously stabilized and controlled at 1,900 F. The boats are kept in the hot zone for about 8 hours, at which time they are moved into the cooler zone and left there for 1 hour at which time they are removed from the furnace. The contents of the retorts are dumped into a coarse screen to separate the coils from the coating powders. The coils are subsequently washed in running water and ultrasonically cleaned and air dried.

Coils prepared by this process have been cyclically tested with the results shown in Table I.

Table I OXIDATION LIFE OF MOLYBDENUM SEALING COILS OF THIS INVENTION 45 Seconds At Temperature 20 Seconds At Temperature 15 Seconds At Room Temperature 12 Seconds At Room Temperature Similarly coated 0.025 inch diameter molybdenum wire are evaluated in a standard 2,500 F ASTM cyclic test (2 minutes on, 2 minutes off) for heater wires and exhibit a life of over hours.

Actual usage of heater coils for the production sealing of diodes indicates an increase in both yield and productivity in comparison with platinum metal coils resulting from the ability of the stronger molybdenum coil to retain its shape, its faster thermal response, and a reduced tendency of the coated coil to stick to the diode glass if they should accidental touch.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

We claim:

1. A process for producing an electrical heating element having a core of molybdenum wire asa substrate and a coating of boron-modified molybdenum disilicide of from about 1 to about 5 mils thickness comprising a. forming a layer of a mixture of boron silicide and sodium fluoride in a container constructed of heat resistant material, 1

b. providing a layer of powdered silicon on said layer of said mixture in said container,

c. placing at least one uncoated molybdenum heating element on top of said layer of silicon in said container,

d. covering said element with powdered silicon,

e. heating said container and its contents in a nonoxidizing atmosphere at a temperature of at least about 1,600" F for at least about 4 hours.

2. A process according to claim 1 wherein the weight ratio of boron silicide to sodium fluoride to said powder silicon is from about 1:100 to about 5:100.

3. A process according to claim 2 wherein the weight ratio of boron silicide to sodium fluoride is from about 1:1 to 1:3.

4. A process according to claim 3 wherein said heating is in a hydrogen atmosphere at about l,900 F for about 8 hours.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,7Lg7 ,207 Dated July 2h, 1973 Inventor-(s) George T. Pepino, at al.

It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 1, line "28, after "heretofore" cancel "glass investment". I

Column 1, line 36, after "the" cancel "lgass" and substitute 1 glass Column l, line-4 5, after "The" cancel "investemnt" and substitute investment E Column 2, line'49', after "a" cancell'freatively" and substitute relatively Column 2,lin'e 57; after- "with" cancel "centainty" and sub- R stitute certainty Column 4, line 64, after "should" cancel- "accidental" and substitute accidentally.

Signed and sealed this 26th day of February 197 (SEAL) Attest:

I I c. MARSHALL BA N EDWARD M.FLETCHERJR' Commissioner of. Patents Atte sting Officer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,714.7,207 Dated July 21+, -973- Inventor(s) George T. Pepino, et 8.1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 28, after "heretofore" cancel "glass investment".

Column 1, line 36, after "the" cancel "lgass" and substitute glass Column 1, line4S, after "The" cancel "investemnt" and substitute investment 7 Column 2, line 49, after "a" cancel "reatively" and substitute relatively Column 2, line 57, after "with" cancel "centainty" and substitute certainty Column 4, line 64, after "should" cancel "accidental" and substitute accidentally-.

Signed and sealed this 26th day of February 197L (SEAL) Attest:

0. MARSHALL DANNM EDWARD Commissioner of Patents Atte sting Officer FORM PC4050 (10-69) USCOMM-DC 60376-PU9 i U. 5. GOVERNMENT PRINTING OFFICE III 0-356-384, o 

2. A process according to claim 1 wherein the weight ratio of boron silicide to sodium fluoride to said powder silicon is from about 1:100 to about 5:100.
 3. A process according to claim 2 wherein the weight ratio of boron silicide to sodium fluoride is from about 1:1 to 1:3.
 4. A process according to claim 3 wherein said heating is in a hydrogen atmosphere at about 1,900* F for about 8 hours. 